It has been predicted that solid-state lighting using light emitting diodes will eventually take over most of the applications now occupied by conventional lighting technology. A major attraction of LED lighting is reduced energy costs due to having inherently greater efficiency than incandescent, fluorescent and high-energy discharge lighting. Other attractions are that LEDs potentially have a much greater life span than the alternatives and do not contain hazardous chemicals such as the mercury used in fluorescent bulbs.
Two current disadvantages of LED lighting are the high cost of the LEDs themselves and the fact that many implementations do not live up to the often-claimed 50K+ hour lifetimes. To address this second issue the driving circuitry sophistication needs to be improved while keeping the cost low and, for practical reasons, the space taken by the controller small. Reliability issues with LED driving circuitry include failures in components such as large electrolytic capacitors used to produce DC voltages for LEDs. Their limited life becomes even shorter as ripple current increases, calling for even larger capacitors. Other contributors to a shorter lifetime are LEDs being stressed by overheating, overvoltage, or current spikes in excess of their maximum ratings. As the price of LEDs comes down the cost of the driving circuitry becomes relatively more important to the total consumer price, but the sophistication of the drive circuit needs to be higher than many circuits currently in use to ensure a long lifetime.
LED current is often regulated with a high frequency switching regulator that uses an inductor and capacitor as storage elements and a flyback diode to recirculate current between switching cycles. Switching regulator circuits are often chosen due to having higher efficiency than most non-switching designs. However, switching regulators have a number of disadvantages that can require additional circuit costs. Switchers create high frequency electromagnetic interference (EMI) that needs to be filtered in order to meet FCC regulations, for example. Also, the switching power supplies can create harmonic distortion in the current drawn from the power line. This is primarily seen as peak currents much greater than the root-mean-square (RMS) current and is drawn primarily at the peak of the AC voltage sine wave due to the capacitive current inrush on each AC cycle. This phenomenon undesirably lowers the Power Factor.
Power Factor is the ratio of real power in watts to apparent power in voltamps (VA). If the effective load of an LED lamp is inductive or capacitive then the Power Factor will be less than the ideal 1.0. Additional circuitry may be needed to correct the Power Factor (PF) of the lamp to meet utility company regulations.
In a lighting system that uses either a switcher or a conventional power supply to produce a DC rail, the PF is typically much less than optimum due to the power supply's input and output filter capacitors. As mentioned, these capacitors draw large peak current near the peaks of the input line voltage and much less between peaks. These distortions show up in the voltage and current frequency spectrums of the system as increased odd harmonics. In the usual lighting installation the power supplied is singlephase 120 VAC or 220 VAC connected phase to neutral. In this case the harmonic distortions will be additive on the neutral and can cause the neutral current to be up to 1.73 times greater than the phase current. This can cause the neutral to overheat even when the load is within the rating of the service. There is a need for circuits for driving LEDs that control the current and do not have inherent EMI and PF problems.